The present invention relates to an inspection method for inspecting a magnetic disk placed in a magnetic disk drive, and more particularly to an inspection method for detecting a physical defect occurring on a magnetic layer of a magnetic disk with high sensitivity without increasing the length of inspection time.
A magnetic disk used for a magnetic disk drive is produced by laminating a magnetic layer on a glass substrate using the thin-film formation technology such as sputtering. Because the glass substrate itself has some unevenness on its surface, or because conditions in a sputtering process sometimes change, a defect specific to a recording medium may occur on a surface of the magnetic layer. The magnetic disk drive has an error correction code (hereinafter referred to as ECC) by which if a bit pattern of data which has been read from the magnetic disk is abnormal, this abnormal condition is detected to correct the read data. Therefore, even if the magnetic disk includes a minute defect, if the number of error bits included in the data to be corrected by the ECC is within a range of the error-correction capability of the ECC, practically no problem is produced.
However, if such a minute defect is left unhandled, the size of the physical defect increases, and consequently there arises a possibility that the number of error bits will increase. In particular, if the magnetic disk is exposed under high-temperature environment for a long time, the defective area further expands. Therefore, in the manufacturing stage of the magnetic disk drive, it is desirable to detect even a minute defect with high sensitivity, and to handle the defect in advance so that the defective part is not used. On the other hand, because it takes a long time to detect a defect, if a magnetic disk drive is produced by means of mass production, pursuing only the improvement in the defect detectivity is not allowed from a viewpoint of the quantity of production or yields. In addition, it is necessary to consider use efficiency of test equipment prepared by a large investment.
A Patent Document 1 (Japanese Patent Laid-Open No. Hei 9-145634) discloses a technology relating to defect detection of an optical recording medium, which enables the defect detection in which the error correction capability of an actually used read/write device is taken into consideration, and which can improve yields at the time of manufacturing disks. Moreover, a Patent Document 2 (Japanese Patent Laid-Open No. 2001-351228) discloses a surface defect test method for testing a magnetic disk medium in which even if there is dispersion in read core widths of an inspection head, there is no dispersion in the sensitivity to a defect on the medium. Further, a Patent Document 3 (U.S. Pat. No. 6,651,213) discloses a technology for determining the redundancy of an ECC in response to the probability distribution of errors for each zone, which is obtained in a manufacturing test stage.